Tensioning apparatus for continuously moving metal strip can be employed in stretch-bending leveling devices for the straightening of metal strip, dressing frames, pickling lines, galvanizing lines or the like. Such apparatus serves to stretch, tension and displace the metal strips and, especially when stretching is desired for strengthening or straightening the metal strip, apply a predetermined stretch thereto during the processing of the metal strip.
The tension is applied to the metal strip generally with the aid of sets bridles of stretching drums or rolls which can be alternately overshot and undershot by the metal strip and which frictionally engage the metal strip, the drums of each set being positioned relative to one another such that the metal band passing around each drum extends through arcs in excess of 180.degree. and preferably of about 225.degree. around each roll or drum. Because of the large surface contact of the drum with the strip, the surface friction is high and special means is not required to maintain the strip in contact with the surface of the drum or prevent substantial slippage between them.
To obtain a predetermined degree of stretch, i.e. elongation, while applying a continuous tension to the band or strip, the rolls are driven with successively higher peripheral speeds and hence, if they are of the same diameter, successively higher angular velocities and can have stepped torques to accomplish this stretching from roll to roll or between steps of rolls.
Between the upstream and downstream sets, a precise torque distribution must be maintained for the individual drums to prevent fluttering of the band or excessive stretching at one location and insufficient tension at another. If the torque distribution is not precise, the stretch of the band does not correspond to the peripheral speeds of the rollers and the desired degree of stretch cannot be maintained.
To compensate for the different angular velocities of the individual drums, in order to ensure operation of the drums at corresponding peripheral speeds as the strip stretches, i.e. corresponding to increasingly band length, various drive and control systems have been proposed for the drums to ensure the precise distribution of the tension force, compensatory angular velocity increases between the upstream and downstream sides of the system, etc.
In one conventional system, a purely hydraulic drive arrangement is provided between the individual stretching drums or rolls. Thus, between the inlet and outlet sides of the system, the drums are provided with respective hydraulic motors and, on the inlet side, these motors function as pumps while at the outlet side, the hydraulic machines function as motors, the high pressure side of the hydraulic machines being connected to a common pressure line whose pressure is maintained or established by a controllable pressurizing pump.
The torques of the hydraulic motors, or, more generally hydraulic machines, are stepped in accordance with the stressing force which must be applied at each such drum. The pumps at the upstream side and the motors at the downstream side are fed in opposite directions (with respect to hydraulic-fluid flow) by the pressurizing pump so that a tension is applied to the strip.
While this system permits predetermined tension forces to be maintained or established, these hydraulic motors and pump arrangements have significant disadvantages.
While the hydraulic motors allow compensation of the angular velocities of the drums to the increase of strip length and can be used as the primary band or strip transport source, for the application of considerable tensions of, for example, to order of thirty million kg-force (30Mp) the hydraulic system is extremely expensive. This is especially the case since the duct work, cooling arrangements, filter devices, pumps and motors must be capable of handling several thousand liters of the hydraulic medium per minute (see for example German Published application -- Offenlegungsschrift No. 16 52 615).
In order to obviate this disadvantage, the applicant has proposed earlier a hydromechanical system which has been described in his commonly assigned U.S. Pat. No. 3,722,776 which issued Mar. 27, 1973. This patent describes an apparatus for the tensioning of continuously displaced metal strip using at least two sets of stressing drums, i.e. at least one pair of rolls at the intake side and at least one pair of rolls at the discharge or downstream side, the band passing alternately over and under the rolls or drums of each set.
In this system, each tension drum is provided with a differential transmission which is constituted as a superimposition transmission, the two transmissions being provided in parallel with one being coupled to the extreme drums or rolls of the two sets while the other is coupled to the inner drums or rolls of the two sets. Motors can be provided to the other imputs of the differential transmissions to effect the necessary speed and torque equalization inputs.
The inputs of these motors is thus superimposed upon the transmissions coupling the corresponding rolls so that the motors can be termed superimposed motors. Since the transmissions serve as means for superimposing an additional drive source on the system, they can be termed superimposition transmissions in the sense required for an understanding of this invention.
The superimposition input of each superimposition transmission is thus connected to one of the superimposition motors to effect torque control.
While this system has been found to be highly useful for the purposes described above, it has required some improvement.
For example, the system has limited versatility and it has not been possible, for instance, to provide the drums in different planes offset from one another or disposed one above another. This limits the use of the system of planes which can accommodate an arrangement of the rolls or drums in a common plane.
Moreover, when the drums are to be operated with small torques, a system having numerous differential transmissions coupling each pair of corresponding drums is relatively expensive.